In a master cylinder, more particularly of the tandem type, each of the primary and secondary pistons is fitted with a moving shutter, or flap, for the opening and closing of a pressure-fluid passage provided in the piston, as a function of the strokes of the piston inside the master cylinder.
Such fluid passage opens axially into a housing formed at one end of the piston and accommodating the flap, which comprises a rod, guided within the fluid passage, and a head situated outside the passage and biased by a return spring towards the fluid passage outlet so as to close it in a tight manner.
Such return spring is a compression spring arranged between the flap head and bearing means, added to the piston.
It has already been suggested that these bearing means be constituted by a metal cage resiliently snapped in the flap housing. For this purpose, the cage comprises resiliently-deformable lugs, situated at its periphery, facing radially outwards and intended to enter and get latched in an annular groove provided in the cylindrical wall of the flap housing. In the course of the assembling process, such lugs are resiliently brought closer to the axis, as the cage is inserted in the flap housing and they rub against the cylindrical wall of the housing till the cage assumes an axial position inside the housing, in which the lugs engage the above-mentioned annular groove in a snapping manner.
The rubbing motions of the steel cage lugs against the cylindrical wall of the housing provided in the piston, made of aluminium or of a light alloy, are likely to scratch the cylindrical wall and tear away metal particles or chips, which may settle under the flap head or on the upper lip of a seal cup, or be the cause of scratches during the piston travel and consequently give rise to tightness failures in the master cylinder, actually a major drawback which it is essential to cure.
Besides, such cage should neither interfere with the brake-fluid flow nor restrict its flow rate.